This invention relates to a stent assembly which comprises a stent and a delivery device for the stent, to a catheter assembly which includes the stent assembly, and to a method of disposing a stent in a lumen in a human or animal body.
Stents are used in lumens in a human or animal body, including for example blood vessels, bile ducts, urinary tracts and so on. When properly positioned in a lumen, a stent can contact the wall of the lumen to support it or to force the wall outwardly.
Stents can be made from a material which enables the stent to be compressed transversely elastically so that they can then recover outwardly when the compressing force is removed, into contact with the wall of the lumen. Such stents are often referred to as "self-expanding stents". The enhanced elastic properties available from shape memory alloys as a result of a transformation between martensite and austenite phases of the alloys make them particularly well suited to this application. The nature of the superelastic transformations of shape memory alloys is discussed in "Engineering Aspects of Shape Memory Alloys", T W Duerig et al, on page 370, Butterworth-Heinemann (1990). Subject matter disclosed in that document is incorporated in this specification by this reference to the document. A principal characteristic of shape memory alloys involves an initial increase in strain, approximately linearly with stress. This behaviour is reversible, and corresponds to conventional elastic deformation. Subsequent increases in strain are accompanied by little or no increase in stress, over a limited range of strain to the end of the "loading plateau". The loading plateau stress is defined by the inflection point on the stress/strain graph. Subsequent increases in strain are accompanied by increases in stress. On unloading, there is a decline in stress with reducing strain to the start of the "unloading plateau" evidenced by the existence of an inflection point along which stress changes little with reducing strain. At the end of the unloading plateau, stress reduces with reducing strain. The unloading plateau stress is also defined by the inflection point on the stress/strain graph. Any residual strain after unloading to zero stress is the permanent set of the sample. Characteristics of this deformation, the loading plateau, the unloading plateau, the elastic modulus, the plateau length and the permanent set (defined with respect to a specific total deformation) are established, and are defined in, for example, "Engineering Aspects of Shape Memory Alloys", on page 376.
Non-linear superelastic properties can be introduced in a shape memory alloy by a process which involves cold working the alloy for example by a process that involves pressing, swaging or drawing. The cold working step is followed by an annealing step while the component is restrained in the configuration, resulting from the cold working step at a temperature that is sufficiently high to cause dislocations introduced by the cold working to combine and dislocations to align. This can ensure that the deformation introduced by the cold work is retained.
The properties of shape memory alloys can also involve thermally induced changes in configuration in which an article is first deformed from a heat-stable configuration to a heat-unstable configuration while the alloy is in its martensite phase. Subsequent exposure to increased temperature results in a change in configuration from the heat-unstable configuration towards the original heat-stable configuration as the alloy reverts from its martensite phase to its austenite phase. It is known from U.S. Pat. No. 5,197,978 to make use of the thermally induced change in configuration of an article made from a shape memory alloy in a stent.
Stents can also be made from materials that do not exhibit the shape memory properties of shape memory alloys. Examples include certain stainless steels.
Self-expanding stents are commonly delivered to a desired location in a lumen using a catheter, in which the stent is constrained in a transversely compressed configuration, from which it can expand when released from the catheter to contact the wall of the lumen. Catheters formed from polymeric material are commonly used, for example because of their flexibility which facilitates steering the catheter through a lumen, and also for reasons of cost. It has been found in certain circumstances that a stent constrained within a catheter formed from a soft polymeric material can become embedded in the internal wall of the catheter due to elastic forces exerted by the stent as it attempts to expand, to an extent which can make it difficult to discharge the stent from the catheter. A stent constraint which is formed from a polymeric material, and which has the physical characteristics appropriate to constrain the stent, will generally have a large wall thickness, making it inflexible and bulky.